"She has worn fur, so the innocent did not get hurt," says Ingrid Newkirk, Peta's founder and director, with a big smile. "It wasn't friendly fire."

The protest made a mess, and a splash. That Saturday there was a spike of 5,000 people playing the "Jennifer Lopez game" on Peta's website. "You try to get animals to escape from J-Lo so that she can't pelt them and make them part of her wardrobe," says Newkirk. "She has no animal consciousness at all. She has mink eyelashes. We have begged her. We have written her nice letters. We have picketed her. We just cannot reach her."

The flour-bombing got big play in the British press, reaching a full range of audiences from the readers of the Daily Star to those of the Daily Telegraph. "We're like a car crash," says 56-year-old Newkirk. "You have to look at us and you'll talk about it afterwards. We can't just deliver the straight facts. People want to be titillated these days - they want sex, they want conflict. It's rather an indictment of what news has become. We're stunt queens. We have to be. We have to escalate so there are more gimmicks than there used to be, because otherwise there is silence and this issue is too important a social issue not to be debated."

So Newkirk gives them what they want. In 1997, Peta activists walked into the Four Seasons hotel in Manhattan and dumped a dead raccoon on the lunch plate of Anna Wintour, the editor of Vogue, saying: "Shame on you, fur hag, this is for the animals." They have sloshed buckets of money soaked in faked blood over audiences at the International Fur Fair and crawled through Paris with leg-hold traps on their feet. In June 2001, Calvin Klein had his black suit plastered with tofu-cream pie. (That was a mistake: "He happened to come into the line of tofu as we were attempting to hit Karl Lagerfeld," says Newkirk.) Last year, Peta campaigner Heather Mills-McCartney lost her prosthetic leg as she clashed with guards while protesting about J-Lo's use of real fur.

Indeed, this desire to either shock or repulse in equal measure is one that Newkirk is determined to take to her grave. Her will dictates that a portion of the flesh in her body will be "used for a human barbecue, to remind the world that the meat of a corpse is all flesh, regardless of whether it comes from a human being or not needed". For similar reasons, she wants her skin to be made into leather products and her feet into umbrella stands.

This may sound extreme, but then to arrive at Peta's US headquarters in Norfolk, Virginia, is to leave the mainstream far behind. In the parking lot, one car bears the sticker, "Powered by tofu". After the receptionist, the first living being I see is a dog poking its nose through a grid set up to stop the numerous pets (which Peta calls "animal companions") in the building from setting off the burglar alarms. In the offices, cats entertain themselves at a series of carpeted play stations set up on filing cabinets and beside desks. While the interview takes place, both Ginger the cat and Missy the dog wander into Newkirk's office.

Newkirk, who would not look out of place at a home counties village fete, was born in England and raised in India. She says she was raised as a voracious meat eater: "When I was growing up we ate everything from calves' brains on toast to mussels that we would pick on the rocks." Her favourite food was liver. She had her first fur coat when she was 19 - a Ginger Rogers squirrel coat. She wore leather shoes until she was 20.

She has never been one to do things by halves. At 22 she was so certain she did not want to have children and would never change her mind that she got sterilised: "I really felt that I knew that so long as there were children who needed homes it was only vanity to have your own."

It was while she was studying to be a stockbroker in Poolesville, Maryland, at about this time, that she woke up to animal suffering. Her next-door neighbour moved and left some cats behind; Newkirk took them to a shelter expecting them to be looked after and was disgusted to later discover they had all been put down.

Shortly afterwards she got a job as a deputy sheriff focusing on animal cruelty cases. One day she had to go to the site of a farm where animals had been abandoned and the only one left alive was a small, emaciated pig. "I was thinking, 'What kind of hideous people left this animal to starve?' That night, I was driving back in the truck and I started thinking what I was going to eat for dinner. I thought, 'Oh good, I've got pork chops.' And the penny dropped."

Since then she has been trying to build her version of a perfect world, this being a place where everyone is vegan, no one wears leather or hunts, and animal companions roam free in the corporate world.

Peta has become renowned for its situationist acts of rebellion, but the context in which its protests take place has changed significantly since Newkirk founded the organisation 26 years ago, working out of her former home in suburban Maryland. The terrorist attacks of 9/11 have gone on to make all kinds of direct-action protest more difficult - if Mills-McCartney can do it, goes the logic, what's to stop a suicide bomber?

In fact, the FBI has been investigating Peta for the past four years. Recently released documents show its interest centred on Peta's involvement with the Animal Liberation Front and the Earth Liberation Front, both of which are listed as "domestic terrorist organisations". Newkirk says she was not surprised to learn about this. The US government, she claims, has been photographing Peta employees coming to work and has asked her to give it the code to Peta's burglar alarms and emergency response numbers. "To paint us as terrorists or linked to terrorists is just a dirty little dishonest trick, because they can't argue the actual arguments," she says.

Given the range of celebrities who back Peta - its star supporters include Alicia Silverstone, Alec Baldwin, Drew Barrymore and Sir John Gielgud - and the millions it receives in donations each year, such dirty tricks, if dirty tricks they be, seem unlikely to get in the way of the charity's activities.

Newkirk is pragmatic about the role of celebrities in the war she is waging. "If I say something, then a few people will take notice," she says. "If Pamela Anderson says something, everyone drops their sandwich and pays attention. Even if they can't stand her or only want to have sex with her, they're all going to listen. So her message penetrates their brains at some level. I always say she has a big chest but she has an enormous heart inside it. She's an activist's activist."

Peta certainly has a far higher profile than, say, campaigns against the war atrocities that have claimed millions of lives in Congo or Sudan. But then, as any Dixie Chick will tell you, some social issues are easier to go out on a limb for than others. Animals don't vote, rarely fight back and, by and large, have voiced no desire for anything that somebody else has got. That makes them easy targets; it also makes them relatively easy to defend without much fear of a backlash. "There's no point in my talking about human beings," Newkirk says at one point. "There are enough people out there talking about human beings." There are millions in Congo and Sudan who might argue otherwise if only they were alive to do so, of course, but Newkirk inhabits a world in which people don't feature much except in regard to what they don't do to animals.

One of Newkirk's employees comes in at one point in the interview with a tiger-skin leotard that she plans to wear in a cage outside a circus later in the week. These stunts, Newkirk says, are not intended to convince but to educate: "We want to make a splash that creates an education. We want them to talk about it and for the discussion to focus on the issue."

But what are the issues? Fur, Newkirk believes, is the thin end of a wedge. What humans simply fail to understand, she says, is that animals are sentient, living beings which should be protected from harm: "Animals need equal consideration. Everybody just has to recognise that they feel pain and that they want to rest in peace. They're not hamburgers on the hoof or a bag waiting to be made up. They're maternal. They have emotions, as we do, and they value their lives, as we do." She describes the animal rights movement as a social movement like those for civil rights, gay rights or feminism; she also compares the treatment of animals to the Holocaust, slavery and the genocide of the American Indians. "We have an exhibit called Holocaust On Your Plate that got us into all sorts of problems because people say, 'You can't make any comparisons to the Holocaust.' But of course you can, because the mind-set that completely disregarded the interests of feeling others, that completely looked down upon and denigrated those others and tortured them and slaughtered them is the same mind-set that has been used throughout history. It was used with lynchings in the South."

When Palestinian terrorists strapped a bomb to a donkey and then exploded it remotely on the road between Jerusalem and the West Bank settlement of Gush Etzion a few years ago, Newkirk wrote to Yasser Arafat asking him to keep animals out of the conflict. (No humans were injured in the attack.)

Talking to Newkirk about this sort of thing makes me want to pull out a stack of baby-back ribs and wolf them down, and then wipe my sticky, saucy fingers all over her desk.

Would she swat a fly? "I might, but I would try not to. But I'm more worried about the top of it than the bottom of it. It takes a lot for people to identify with insects. But it takes nothing, surely, for people to understand that cow mothers are good mothers, that they love their calves, that a slaughterhouse is a vile and ugly place for them."

Newkirk can be maddening, but just occasionally she hits you with a point that makes you really listen. "People in their own time can look back and they can so readily condemn the atrocities of the past," she says. "There's no trick to it. We all know there were atrocities of ethical blindness and a lack of empathy and compassion. Here's the trick. What am I doing today? What blind eye am I turning today to the suffering of others that I am causing?".